This invention generally relates to magnetic disk memories and more specifically to a transducer positioning system for such disk memories.
A data processing system generally comprises one or more secondary storage facilities. Typically, such a facility includes a controller and one or more drives. This invention is particularly adapted to such a facility in which the drive comprises a magnetic disk medium for storing information on plural concentric data tracks.
A typical magnetic disk drive includes transducers, or read-write heads, and related control circuitry for transferring information from and to the media. The specific control circuitry varies, especially between two major categories of magnetic disk drives: namely, fixed head and movable head disk drives. In a fixed head drive, one transducer is positioned at each data track, whereas a single set of heads moves from track to track in a simple movable head drive. The duplication of heads in a fixed head drive is expensive. In some applications, however, the expense is justified because there is no "positioning" delay; i.e., the delay encountered while a head moves between tracks in a movable head drive. However, movable head drives are considerably less expensive because they eliminate head duplication. In most applications the inherent positioning delay is acceptable. This invention is applied to such movable head drives.
One of the important elements of a movable head drive is the servo system for locating, with accuracy, the heads over the data tracks. In large disk drives containing a number of different disks or records, corresponding data tracks on each record are in alignment and form a "cylinder". Normally, a photoelectric servo system controls the movement of the heads from one cylinder to a designated cylinder during a "seeking" operation. Another servo system uses servo information stored entirely on one servo surface of one record for "positioning" all the heads on the designated cylinder. The dedication of one surface to servo information can be tolerated in large drives as the percentage of available storage space used for storing data is still very large.
When the foregoing approach to transducer positioning is applied to small disks including one or two records, inefficiencies result. For example, a dedicated servo surface may consume 25% to 50% of the available storage space. Moreover, the cost of a photoelectric servo system assumes a significant portion of the overall disk cost. The net result is an increased cost per character of available storage, which is the primary economic measure of any disk drive. Several general categories of alternate positioning systems have been proposed to reduce these costs. In one category the disk drives use "in data servo information"; that is, each data track on the disk contains both data that is recorded in spaced data sectors and servo information that is recorded between the data sectors.
In one such drive, course and fine servo information is recorded sequentially between the data sectors. The fine servo information utilizes transitions to discriminate odd and even tracks and is used during positioning to maintain the heads on a designated track. For seeking during which the heads move from one track to another, three cells in each block are encoded to discriminate one of eight tracks. As the heads moved to a new track, these cells are decoded to determine the distance moved. The control circuitry decrements a difference counter depending upon the contents of those cells. In this approach, the servo data is offset by one-half track with respect to the data itself.
In another disk drive, first and second blocks of servo information are recorded across six tracks between the data sectors. There is no phase difference between the signals recorded in the blocks on adjacent tracks. Discrimination depends upon the "length" of the servo information block along the track. Thus, the control circuitry must precisely measure the interval for each block in order to identify its position.
The foregoing approaches require either a complex recording procedure and/or inclusion of complex and expensive decoding circuitry in order to provide accurate servo information. In another approach each track is divided into half tracks and contains circumferentially spaced blocks of servo information alternately recorded in each halftrack. The control circuitry measures and compares the amplitudes of the two blocks to determine whether the heads are centered on the track. It is necessary to move the heads quite slowly during seeking in this approach. If the heads cross more than one track during an interval between successive servo blocks, an ambiguity results because there is no inherent track identification in the servo information. Although this approach is easy to implement, its inherent slowness often leads to intolerable positioning delays.
Therefore, it is an object of this invention to provide a magnetic disk memory that utilizes in-data servo information in an efficient manner.
Another object of this invention is to provide a magnetic disk memory in which in-data servo data information allows the efficient implementation of both positioning and seeking operations.
Still another object of this invention is to provide a magnetic disk memory unit that can be constructed for a relatively low cost;
Yet another object of this invention is to provide a low cost magnetic disk memory device that is reliable in operation.